The following patent publications are believed to represent the current state of the art:
U.S. Patent Publication Nos.: 2009/0084840; 2006/0293929; 2006/0131390, 2006/0131385; 2010/0030592; 2009/0281904; 2005/0203854; 2005/0256806 and 2004/0030659; and
U.S. Pat. Nos. 7,032,823; 7,392,388 and 6,736,322.